Elongated organ associated with the mouth. For example, in elephants, the trunk is the proboscis, while in butterflies, the long, coiled feeding tube is the proboscis.

Having a chemical composition close to the original composition of the material from which the Solar System formed. Some samples from the solar wind or from comets have compositions that are close to primordial and can tell us about the early history of the Solar System.

See bottleneck.

A node on a phylogeny where more than two lineages descend from a single ancestral lineage. A polytomy may indicate either that we don’t know how the descendent lineages are related or that we think that the descendent lineages speciated simultaneously. For a more detailed explanation, see our resource on polytomies in Evolution 101.

Term used to describe to a group of organisms that does not include the most recent common ancestor of those organisms.

The number of copies of each chromosome an organism carries. For example, humans are diploid (i.e., we have a ploidy of two) because we carry two copies of each chromosome.

A situation in which a single gene influences more than one trait or has more than one phenotypic effect. For example, the gene that causes sickle cell anemia affects the shape of red blood cells, blood flow, tendency towards fatigue, malaria resistance, and many other traits.

A broad theory that uses movements of continental plates to explain many geographic, geologic, seismic, and even biological observations. The idea is that the Earth’s crust and upper mantle are made up of many differently sized and irregularly shaped plates that “slide around” on the lower mantle. The plates may crash into one another, slide under one another, and change shape as they are broken down and reformed.

A mammal, such as a human, whose young completes its embryonic development in the uterus, joined to the mother by a placenta.

In placental mammals, the organ that connects a fetus to the wall of its mother’s uterus. Nutrients and oxygen pass through the placenta from the mother to the developing embryo and waste products pass back through it into the mother’s bloodstream.

Substance that absorbs light. Pigments absorb light of particular wavelengths, which gives the pigment a characteristic color.

A system of classification that names groups of organisms according to their evolutionary history. Like Linnaean classification, phylogenetic classification produces a nested hierarchy where an organism is assigned a series of names that more and more specifically locate it within the hierarchy. However, unlike Linnaean classification, phylogenetic classification only names clades and does not assign ranks to hierarchical levels.

A principle stating that the simplest explanation accounting for the observations is the preferred explanation. When reconstructing the evolutionary relationships among lineages, the principle of parsimony implies that we should prefer the phylogeny that requires the fewest evolutionary changes.

Term used to describe a group of organisms that includes the most recent common ancestor of all of its members, but does not include all of the descendants of that most recent common ancestor.

The study of fossils.

A scientist who studies fossils (paleo = ancient, onto = being, ology = study of; study of ancient beings).

A paleontologist that studies fossils of humans and their closest relatives.

Having some features of the ancestral juvenile stage, but being an adult (with a mature reproductive system). This word means “child form,” and a paedomorphic change is any evolutionary change in the development of an organism that generates an adult with a “child’s form.”

A rotating disk of gas and dust surrounding a young star. Our own Sun and Solar System formed from a protoplanetary disk.

The study of how allele and genotype frequencies in a population change over time through the action of genetic drift, natural selection, mutation, migration, recombination, and reproduction. For more details, see our historical essay on the topic.

Generally, a group of organisms living close to one another that interbreed with one another and do not breed with other similar groups; a gene pool. Depending on the organism, populations may occupy greater or smaller geographic regions.

Organism that lives on or within another organism, on which it feeds

The physical features of an organism. Phenotype may refer to any aspect of an organism’s morphology, behavior, or physiology. An organism’s phenotype is affected by its genotype and by its environment.

Organism killed for food by a predator.

An organism that hunts and eats other organisms. Predators may eat plants or meat.

Degree to which an organism’s phenotype changes depending upon its current or past environment. Two organisms with the same genotype (e.g., identical twins) may have different phenotypes (e.g., one may be taller or heavier) if raised in different environments; those differences represent phenotypic plasticity. All organisms exhibit some degree of phenotypic plasticity (e.g., an animal that receives more food will generally be heavier than a genetically identical animal that receives less food), but sometimes phenotypic plasticity can be extreme (e.g., some fish become either male or female depending upon the temperatures they were exposed to as an egg). For more details, see our news story on the topic of phenotypic plasticity.

The ancestral character state for a particular clade. This character state may change depending on the clade under consideration. For example, “has four legs” is plesiomorphic for the clade of terrestrial vertebrates, but “has two legs and two wings” is plesiomorphic for the clade of owls.

The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the “family trees” of groups of closely related species, but we can also use a phylogeny to depict the relationships between all life forms. For a more detailed explanation, see our resource on phylogenies in Evolution 101.

A molecule made of a string of amino acids, folded into a complex three-dimensional structure. Proteins are coded for by DNA and are essential molecules for life.